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Heat
and Temperature
HEAT Heat is a
form of internal energy which is transferred from one object to another due to
a difference in temperature between the objects. Heat is the total energy of
motion of all particles (the total kinetic energies of all the particles.)
TEMPERATURE The temperature of a body of matter is a measure of the
average kinetic energy of the random motion of its particles. Temperature is
the kinetic energy divided by the number of particles. Temperature is that
property of a substance which determines whether it is in thermal equilibrium
with another object.
THERMAL EQUILIBRIUM
This is the situation
in which no heat moves from one object to another.
CALORIE A
15° Calorie is the amount of heat energy needed to change the temperature of
of 1 gram of water by 1° C (from 14.5° C to 15.5° C at 1 atmosphere of
pressure). 1 calorie = 4.185 Joules and 1 kilocalorie = 1000 calories. SPECIFIC
HEAT CAPACITY This
is the amount of heat (in calories or Joules) that must be added or removed
from a unit mass of that substance to change its temperature by one degree.
Different substances have different capacities because they absorb and release
heat at different rates.
WATER Water has a specific heat capacity of 1.00 cal/g °C or 4.185
Joules/g °C. The SI unit would be 4185 J/kg °C.
PRINCIPLE OF HEAT
EXCHANGE The heat lost by an object must equal the heat gained by
the object to which the heat is transferred. There must be a temperature
difference for heat to be transferred.
Q (heat energy) = m (mass) x At (temp.) x cp (specific heat
capacity)
Problems: 2)
A calorimeter contains 300 grams of water at 10° C. After a food sample is
burned in the calorimeter the water temperature changes to 15° C. How much
heat was given off by the food sample? LATENT
HEAT Latent
heat is the heat required to bring about a change in state. HEAT
OF FUSION The
heat of fusion is the amount of heat that must be supplied to change a unit
mass of the substance at its melting point from solid to liquid. The heat of
fusion of water is 80 calories per gram (80 kcal/kg). HEAT
OF VAPORIZATION
The heat of vaporization is
the amount of heat that must be supplied to change a unit mass of the
substance at its boiling point from liquid to gas or vapor state. For water
this is 540 cal/g or 540 kcal/kg.
HEAT
OF SUBLIMATION The
heat of sublimation is the heat needed to change a solid to a gas.
HEAT
OF CONDENSATION The heat of condensation is the reverse of
the heat of vaporization, it is the heat given off when a gas condenses to a
liquid.
Transfer
of Heat by Conduction, Convection, Radiation
Conduction is
a consequence of the kinetic behavior of matter. Faster vibrating particles
collide with less energetic neighbors and transfer some of their kinetic
energy to the slower moving particle.
Through successive molecular collisions energy travels through a
material without the average position of the particles being changed. There
must be a temperature differential (one end of some object at a higher
temperature than the other) for heat to be conducted.
Gases are poor conductors of heat (compared to liquids and solids)
because the molecules are relatively far apart and collisions are infrequent.
Metals have the greatest ability to conduct heat (for the same reason as
their high electrical conductivity). This is due to a significant number of
electrons being able to move about freely instead of being bound permanently
to particular atoms.
Thermal conductivity
of a material is a measure
of its ability to conduct heat.
Example: wood and metal Convection
involves the actual motion
of a hot fluid from one place to another, displacing a colder fluid in its
path and setting up a convection current. Convection is the chief mechanism of
heat transfer in fluids.
Natural convection occurs when the buoyancy of heated fluids leads to
motion. Heated fluids (gas or liquid) expand and becomes less dense than
surrounding cooler fluids. It then rises.
Radiation is
defined as the energy that is transmitted by electromagnetic waves and
requires no material medium for passage.
All objects radiate electromagnetic waves with the higher the
temperature of an object the shorter the predominating wavelength of its
radiation.
Example: see glass lined thermos bottle in heat packet in class.
Thermal
Expansion of Water
Ice floats (less dense than the water).
A body of water freezes from the top down.
Ice is such a poor conductor of heat that this initial ice layer impedes
further freezing.
This allows fish and plant life to live through the winter.
The spaces between molecules in ice are greater than the same spaces in
liquid water.
Ice has what is called an open structure. Each H2O molecule
bonds with 4 other H2O molecules while other solids can have
molecules with as many as a dozen bonds with surrounding molecules resulting
in a compact substance.
The density of water increases from 0° C to 4° C (as the volume
decreases). Large clusters of H2O molecules break into smaller
clusters that occupy less space in the aggregate as the temperature rises to
the 4° C mark. The greatest density of water is at 4° C.
Above 4° C , the normal thermal expansion of materials is seen. Here as
the temperature rises the density decreases.
Four
States of Matter
Matter is
defined as any material that has mass, occupies volume, and exhibits inertia
(resistance to movement).
Solids
definite shape and volume, resist deformation
Crystalline solids
particles
are arranged in regular, repeated patterns - said to have long-range order to
their structure -example would be NaCl (table salt)
Amorphous solids
solids that lack the definite arrangement in crystals are
`amorphous' which means `without form'
Liquids
definite volume, resist compression, will flow, takes the shape of its
container
Gases
Have no definite shape or
volume, takes the shape and volume of its container
Plasma
very high
temperature ionized gas (as high as 100 million degrees in some fusion
reactors)
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Page Last Updated: Friday March 02, 2007 Webmaster: Larry Jones Pickens County School District |
